Can a seed bank maintain the genetic variation in the above ground plant population?

Abstract

There are indications that a persistent seed bank can protect small and isolated plant populations from local extinction. Genetic mechanisms contributing to this phenomenon are the increase of local effective population size - and hence the decrease of genetic drift - through a reservoir of persistent seeds, and the accumulation of intergenerational genetic diversity in the seed bank. To find evidence for these mechanisms, we conducted two formal meta-analyses. First, we analyzed 42 published habitat fragmentation studies and investigated whether the degree of genetic differentiation between fragmented plant populations was mediated by seed longevity. Second, we reviewed 13 published studies reporting the genetic diversity of both the seed bank and the above ground plants, aiming at comparing genetic diversity contained in the seed bank with the above ground vegetation. We conclude that a persistent seed bank may indeed mitigate the consequences of habitat fragmentation and protect a species from genetic drift and population genetic differentiation. We found no evidence, however, of high levels of genetic diversity accumulating in the soil seed bank. If genetic differences are present between the standing crop and the seed bank, they are very likely the result of local selection acting either directly or indirectly as a filter on the alleles present in the seed bank. We finally suggest that 1) the role of the seed bank should not be neglected in habitat fragmentation studies and 2) it is not very fruitful to continue comparing seed bank genetic diversity with above ground plant genetic diversity, unless this is performed under different selection regimes

Inference of seed bank parameters in two wild tomato species using ecological and genetic data

“…It has been shown that S. chilense has shorter dormancy in stress conditions (i.e., low temperatures), than S. peruvianum (32,33), and variability for dormancy is a heritable trait in these plant species (18) as well as in others (17). Combining these results, we suggest that (i) S. peruvianum may occur in habitats that are more variable across years than those of S. chilense, although temporal variability of climatic conditions has not yet been included in ecological studies (25,30), or that (ii) geographic variability of habitats may promote a longer seed dormancy in S. peruvianum as a generalist strategy to colonize different demes/habitats (13) and reduce risks for extinction following the temporal rescue effect (15). On the other hand, these two wild tomato species differ in their life cycle, with S. chilense being characterized by a generation time of 3 to 7 y (29, 30, 34) and S. peruvianum being characterized by a more variable generation time of 1 to 7 y (34).…”

“…The mutation rate is not thought to increase in the seed bank with the age of seeds (8,15). The model parameters are provided in SI Appendix, Table S8.…”

“…Moreover, evidence for seed banks to increase the observed molecular diversity compared with expectations from the above-ground census size (15,16) can be confounded by the spatial structuring of populations, where a significant part of the diversity may be attributable to population differentiation. Indeed, in a Wright-Fisher model of reproduction, both seed banks and spatial structure can be seen as a similar departure from random mating because there is separation of individuals into either different age classes (19) or different spatial demes (20).…”

“…First, low germination rates in the banks promote the storage of genetic diversity and the increase of N e compared with above-ground plant populations (6)(7)(8), although slowing down the rate of evolution (5) and coevolution (14). Second, seed banks counteract habitat fragmentation by buffering against the extinction of small and isolated populations, a phenomenon known as the temporal rescue effect (15).…”

Cryptic genetic subdivision in the San Benito evening primrose (Camissonia benitensis)

“…These populations may still harbor undetected genetic variation. For example, additional alleles may be hidden in the seed bank (Honnay et al 2008), which has been conservatively estimated as averaging 5199 the size of the standing populations (BLM 2011). Our sampling included field collected plants and plants germinated from the seed bank, and there were slight differences in the distribution of genetic diversity (field collected F IS = 0.868 ± 0.06; seed bank F IS = 0.780 ± 0.10) and significant population differentiation between field and seed bank samples (F ST = 0.072; P \ 0.0001).…”